The present invention relates generally to identification of unknown members of a sample by mobility characteristics, and more particularly to devices that analyze compounds via field-based ion mobility spectrometry.
There are a number of different circumstances in which it is desirable to perform a chemical analysis to identify compounds in a sample. Such samples may be taken directly from the environment or they may be provided by specialized front-end devices that separate or prepare compounds before analysis.
Furthermore, recent events have seen members of the general public exposed to dangerous chemical compounds in situations where previously no thought was given to such exposure. There exists, therefore, a demand for accurate, easy to use, and reliable devices capable of detecting the chemical makeup of a sample rapidly and even at trace levels, whether in the laboratory or in the field.
Mass spectrometers are generally recognized as highly accurate detectors for compound identification, given that they can generate a fingerprint pattern for even fragment ions. However, mass spectrometers are quite expensive, easily exceeding a cost of $100,000 or more and are physically large enough to become difficult to deploy everywhere the public might be exposed to dangerous chemicals. Mass spectrometers also suffer from other shortcomings such as the need to operate at relatively low pressures, resulting in complex support systems. They also need a highly trained operator to tend to and interpret the results. Accordingly, mass spectrometers are generally difficult to use outside of laboratories.
A class of chemical analysis instruments more suitable for field operation operate based upon aspects of ion mobility in an analytical field. One such type is known as Field Asymmetric Ion Mobility Spectrometers (FAIMS) (also known as Radio Frequency Ion Mobility Spectrometers (RFIMS) and Differential Mobility Spectrometers (DMS), among other names. This type of spectrometer subjects an ionized sample to a compensated varying high-low asymmetric electric field and filters ions based on aspects of their field mobility.
Typically a gas sample flows through a varying high asymmetric RF field which allows only selected ion species to pass through, according to an applied low DC compensation voltage, and specifically only those ions that exhibit selected mobility responses in the field. An ion detector then collects detection data for the detected ions. This may include intensity data shown as detection “peaks.” These peaks are interpreted according to the compensation voltage at which a species of ion is able to pass through an asymmetric field of given field parameters.
A typical FAIMS device includes a pair of electrodes in a drift tube. An asymmetric field is applied to the electrodes transverse to the ion flow path. The asymmetric RF field alternates between a high or “peak” field strength and a low field strength. The field varies with a particular frequency and duty cycle. Field strength varies as the applied voltage and size of the analytical gap between the electrodes.
In a FAIMS device, ions will pass through the analytical gap between the electrodes only when their net transverse displacement per period of the asymmetric field is zero; in contrast, ions that undergo a net displacement will eventually undergo collisional neutralization on one of the electrodes. In a given RF asymmetric field, a displaced ion can be restored to the center of the gap (i.e. compensated, with no net displacement for that ion) when a low strength DC electric field (the compensation voltage, Vcomp) is superimposed on the RF. Ions with differing displacement (owing to characteristic dependence of mobility in the field) can be passed through the gap at compensation voltages characteristic of a particular ion, which is accomplished by applying various strengths of Vcomp. With Vcomp held at one value, the system can function as a continuous ion filter, or a scan of Vcomp will allow complete measure of the spectrum of ion species in the sample. The recorded image of the spectral scan of the sample is sometimes referred to as a “mobility scan” or as an “ionogram”.
The detected compounds can be identified by comparing detection data against a library, for example, of stored known identification data. By noting the RF level and compensation voltage and the corresponding detected signal, an ion species can be identified, as well as concentration level (as seen in the detection peak characteristics). Ideally a specific RF level and compensation voltage will permit only a particular species of ion (according to signature mobility in the field) to pass through the filter to the detector. However, if mobilities overlap under the selected field conditions, then the detected species may contain ions for several compounds happening to have the same mobility under those conditions. This can result in detection errors, sometimes referred to as “false positives”.
It is an object of the invention to provide a simple and compact apparatus to achieve such detections with improved accuracy.
It is another object of the invention to provide an improved ion species detection device with higher sensitivity and reduced false positives.